Since his fame is relatively new and hasn't gone to his head (not that his dad Cecil would let it) Prince doesn't charge for autographs. As far as the potential worth goes, it depends on what his stats continue to do, and how well the Tigers do this year. GO TIGERS!!!
Autographs are usually free when given by the celebrity. If you are, however, asking about autographed items, I say it depends on the item and sometimes the year when it was autographed. You can get an appraiser to give you a precise quotation of the price.